1. Field of the Invention
The present invention relates to an acrylic syrup and to a method of producing the same. More specifically, the invention relates an acrylic syrup adapted to producing acrylic resin molded articles having excellent impact resistance and, particularly, to producing artificial marble and to a method of producing the acrylic syrup.
2. Description of the Related Art
An expansion in the market of acrylic resins in recent years is accompanied by a development of use of the acrylic resins as interior construction members, such as the use as an artificial acrylic marble.
The artificial acrylic marble is usually produced by mixing a polymerizable viscous liquid (acrylic syrup) comprising a methyl methacrylate as a chief component with an inorganic filler such as aluminum hydroxide, a crosslinking agent, an ornamental member and a polymerization initiator, polymerizing and solidifying the mixture into a plate by a cell-casting method or a continuous casting method, and cutting the plate into a desired size. Concrete examples of use of the artificial acrylic marble include kitchen counter, washstand, dressing table, bathtub, floor member and wall member.
The artificial acrylic marble is highly evaluated for its high-quality appearance and feasibility of design involving, however, such defects as being brittle and weak against shocks. Products produced from the artificial marble are likely to be broken during the transit, working, assembling or use and, hence, a countermeasure must be taken for preventing the products from being broken by shock.
If the impact resistance of the artificial marble could be improved, then, inconvenience involved in the handling such as during the transit of articles and during conducting various workings can be decreased and, besides, the thickness of the articles can be decreased. It is therefore very meaningful to develop an artificial acrylic marble having excellent impact resistance compared to that of conventional artificial acrylic marbles.
In order to impart impact resistance to the resins, it is widely accepted practice to modify the resins with a rubbery polymer. As the thermoplastic resins imparted with impact resistance by modification, there have been widely known an ABS resin and an HIPS resin which are produced by dissolving a rubbery polymer (hereinafter often referred to as rubbery component) in a monomer followed by polymerization, i.e., by graft-polymerizing the rubbery component with a monomer such as acrylonitrile or styrene. In order to improve impact resistance by the modification with a rubbery component, further, it is necessary that the rubbery component that is grafted (grafted rubber) is dispersed in the form of particles in the resin. For example, when the mass polymerization is conducted by using a monomer solution of the above-mentioned rubbery component, it has been known that the phase separation occurs as the monomer solution is polymerized and that the grafted rubber disperses in the form of particles due to the phase conversion as a certain polymerization conversion is exceeded. The shape and size of the thus dispersed grafted rubber particles affect the impact resistance, transparency and workability of the product resins. The ABS resin or the HIPS resin produced by the mass polymerization forms a special micro-structure in which a resin phase is contained in a rubber phase.
An attempt for improving the impact resistance by using the grafted rubber as described above, has also been conducted in the production of acrylic resins through an acrylic syrup. For example, Japanese Examined Patent Publication (Kokoku) No. 25215/1980 discloses a method of producing a rubber-reinforced acrylic resin by dissolving a rubbery polymer in a monomer which comprises a methyl methacrylate as a chief component, synthesizing a rubber-reinforced acrylic syrup by the batchwise mass polymerization, and removing the residual monomer under a reduced pressure. As the rubbery polymer, there is used a material which is rubbery at room temperature and is graft-copolymerizable with a monomer, such as a polybutadiene or a butadiene/styrene copolymer.
When the rubber-reinforced acrylic syrup is synthesized by the above-mentioned method, however, it becomes difficult to control the reaction since the polymerization initiator is added at one time in an amount necessary for the reaction. That is, abnormal acceleration of polymerization called gel effect becomes very conspicuous, and the reaction system becomes out of control, so that the content in the reaction vessel is quickly solidified or the polymer adheres on the inner walls of the reaction vessel and gradually grows thereon and turns into insoluble polymer causing the interior of the reaction vessel to be clogged, making it no longer possible to stably conduct the operation. Further, since the reaction is conducted under a reflux condition by raising the temperature after the polymerization initiator is once added, it becomes difficult to stably produce the products being affected by the rate of elevating the temperature, reflux amount and even by a slight change in the temperature.
In order to improve the above-mentioned problems, Japanese Unexamined Patent Publication (Kokai) No. 147514/1980 discloses a method of producing a rubber-reinforced acrylic syrup by the continuous flow-type mass polymerization. According to this production method, a rubber-reinforced acrylic syrup is obtained by continuously feeding a starting polymerization material obtained by dissolving a rubbery polymer in a monomer which comprises a methyl methacrylate as a chief component and feeding a starting monomer in which a radical polymerization initiator is dissolved, into a single reaction vessel, and by conducting the polymerization while continuously stirring the solution and controlling the temperature to lie from 90° C. to 200° C. and controlling an average residence time to be from 0.5 to 30 minutes. As the rubbery polymer, there is used a polybutadiene, a butadiene/styrene copolymer, a butadiene/acrylonitrile copolymer, an ethylene/vinyl acetate copolymer or a polyurethane. According to this method, however, the reaction vessel is fully filled with the liquid at all times arousing such a problem that the polymer adheres and gradually grows on the inner wall surfaces of the reaction vessel. Besides, since the polymerization temperature is high, a copolymer of a low molecular weight is formed much. As a result, if the acrylic resin obtained from such an acrylic syrup is molded, the resin is discolored and is molded defectively.
Further, Japanese Unexamined Patent Publication (Kokai) No. 302010/1997 discloses a method of decreasing the volume contraction factor during the curing in the production of an acrylic resin by adding a rubbery polymer to an acrylic syrup, and by dispersing the rubbery polymer in the form of particles having particle diameters of from 5 nm to 800 μm while stirring the acrylic syrup at a stirring speed of about 10,000 rpm by using a high-speed stirrer. According to this method, the rubbery polymer is dispersed in the acrylic syrup but has not been grafted (or the rubbery polymer is grafted very little even if it is grafted) offering an advantage of decreasing the contraction factor during the curing but without being capable of improving the impact resistance. This is due to the fact that the rubbery polymer particles have a low interfacial adhesion property permitting the interface to be peeled off. Further, the rubbery polymer dispersed on the surface of the acrylic resin precipitates causing a decrease in the surface hardness of the resin.
When the rubbery polymer is simply dissolved in the acrylic syrup, an extended period of time is required before it is completely dissolved, and the acrylic syrup and the rubbery polymer are gradually separated from each other and become inhomogeneous when the acrylic syrup is left to stand still, thus arousing a problem in the preservation stability of the syrup. When, for example, an acrylic resin plate is prepared by using a syrup in which the rubbery polymer is dissolved, the impact resistance is not at all improved and, besides, the surface hardness decreases as compared to that of the article to which no rubbery polymer is added.
As described above, though there has heretofore been proposed a method of producing a rubber-reinforced acrylic syrup adapted to the production of an acrylic resin having improved impact resistance relying upon a batchwise mass polymerization method or the continuous flow-type mass polymerization method, the impact resistance of the acrylic resin finally obtained has not yet been improved to a satisfactory degree. Besides, there has not yet been developed, either, a method of stably producing a rubber-reinforced acrylic syrup of a stable quality by using a simple apparatus.